Are we prisoners of our past? Or free agents, able to blithely reinvent ourselves and embrace tomorrow's bright new dawn? Philosophers, theologists and psychologists have wrestled with these issues for millennia, and there are no easy answers. The truth is, surely, that we are a mix: partly the creatures of our past experiences, but also able to learn from these and move on? That is not to diminish the continuing controversy around such topics as nature versus nurture, but there are certain assumptions we need to make in order to reach a better understanding of ourselves and others.

First, that we are, to an extent that varies for each of us, constrained by our own histories. This comprises our upbringing, but also our own choices and the hand we have been dealt by life. We can further assume that changing these entrenched patterns will not be easy, but it is possible.

Second, we can assume that our character, and therefore our potential, is partly genetic but also partly environmental. That is, some parts of our psychic make-up are determined by our DNA while others are shaped by our parents and childhood. The latest thinking sees these factors not as opposing alternatives but as interacting dynamics that affect one another.

In other words, while even the most extreme genetically determined dispositions tend to make us more likely to develop a characteristic, they rarely doom us to it.

This was dramatically shown in an experiment where the calmest newborn babies in a group of monkeys were given to adults who seemed most anxious and ill-equipped for mothering. The most anxious looking babies were, conversely, transferred to the mothers researchers had identified as being the calmest, most engaged parents. Within a few days, some of the hitherto placid babies were visibly disturbed, and most of the ones who had appeared anxious had begun to calm down. The environmental impact of mothering had begun to interact with the babies' innate temperaments (which must have been largely determined by genes).

Finally, we must assume that, without minimising how hard it can be,peoplelike monkeys - can change. By my late 20s, I had turned into a pretty emotionless hedonist, partying a lot, taking drugs and seemingly incapable of fidelity. I am today, in those respects, a different person.

But some of my old self remains: I can still be selfish, obsessive and demanding. I have come to the conclusion that it is the scale and complexion that changes. Crucially, what has altered fundamentally is my ability to empathise. I take others into account now in a way I never did before: that alone transforms my behaviour. I may be tempted to behave a certain way, but I easily resist. Other things matter more.

Theories of change

But, even if you buy into these assumptions, how can they translate to a deeper understanding of yourself and others, let alone achieving meaningful change in your life? By looking at three key psychological thinkers, we can see how they believed our past helps create and constrain us, and what their theories point to in terms of breaking free of those chains.

Inevitably, we must start with Sigmund Freud. When I was first assigned his works during my training, I was as sceptical as anyone else about his theories. Even now, some, such as penis envy, continue to seem ridiculous. But if you read him in the original, you will be blown away by the depth and breadth of his insight. A key plank of his work suggests that the struggles we go through as babies determine much of how we relate to the world as adults.

Freud believed that these conflicts lodge deep in our unconscious minds, emerge as dreams and can be treated best by intensive psychoanalysis - a form of therapy he was instrumental in developing.

Much of this came to be challenged, but the idea that we have, imprinted upon us from the earliest age, a sense of who we are and what we can become, remains potent.

Coming several decades after Freud, and shaking up the psychoanalytic establishment he created, was an Englishman named John Bowlby. He is now renowned for his work on attachment, which informs much modern psychotherapy and parenting advice.

Bowlby's critique of Freud was that he didn't pay enough attention to what parents contributed to a child's early struggles and development.

He believed that the way a mother (and father to a lesser extent, in those times at least) relate to their baby is paramount. Do they offer a stable, secure, loving attachment, or an instable, anxiety-ridden form of care that, at its most extreme, can create real mental illness? What Bowlby made us realise is that, if we have suffered some failing in attachment, we can repair that in new relationships, either with others or, if need be, in therapy.

I see this played out in my consulting room. Often, two people who have suffered similar traumas in childhood can cope completely differently. Some of that, it's true, can be down to their in-built levels of resilience. But often I find that it is down to something else: despite their parental deprivation they had someone else to hold on to. One man I treated became very close to a neighbour, who offered him fathering that was missing in his chaotic home. A woman I saw for several months had met a stable, dependable man at college who she went on to marry, and who seemed to be able to help right the wrongs of her past. Her sister had gone from bad adolescent relationships to bad adult relationships and was much more depressed and dysfunctional.

A working model

Bowlby called the template that these early relationships lay down a "working model". Other therapists call the same thing a "script" or "core belief". We all have one: for example, "Everyone I love abandons me", or "I am always the needy one".

A strong first step towards better emotional health is identifying yours and working to transform it. This idea was developed best by the father of cognitive behavioural therapy (CBT), Aaron Beck.

Beck and his modern adherents believe that delving into the past, like Freud once did, is unnecessary, and that working on changing today's thoughts and feelings can be transformative. The following exercise, which demonstrates CBT in action, aims to identify your internal critic - that part of you that is hostile and undermining - and replace him or her with an ally. To do this, Beck believes, you must focus on all of the thoughts and half-thoughts that flit across your mind every day in which you put yourself down, even if they seem light-hearted and humorous. Note these down over the course of a whole day. Here are some examples of what might be happening and your response to the situation:

• I've lost that telephone number I need. I'm an idiot, I'm always losing things.

• My boss has asked me to correct a mistake.Why do I always get it wrong?

• I didn't pick up that parcel from the Post Office. I'm so lazy.

People almost always see a clear pattern in their self-criticisms. They congregate around one or two basic themes. This means you can develop an all-encompassing "umbrella" mantra that will challenge these criticisms. Your main ones will probably be obvious, but if they aren't just pick one theme that seems to stand out, for example, "I'm unreliable", and work on that. Here are three common examples, and positive mantras to go with them:

• I'm not good enough. I am good enough, and sometimes I can be great.

• I always get it wrong. I'm fallible, like everyone, but often I get things right, too.

• I am useless. I sometimes need to work harder, but I'm proud of what I do well.

Every time you become conscious of that negative critical script running through your mind, consciously challenge it with your positive mantra. If you do this dozens, and then hundreds, of times, it should start to become embedded in your mind. A new, more realistic, supportive ally will have taken the place of your critic.

CBT helped me, early on in my therapeutic journey, but in my own practice I like to use a combination of the detailed work on changing thinking patterns, alongside an exploration of the past.

Your past self

A final exercise that can be helpful in understanding how you got to where you are today is to visualise your past self. Focus on the long list of critical thoughts and think about when you first started to hear and believe such things. Conjure up a mental image of you at that age. See if you can gauge the expression on this "inner child's" face. Is she or he happy? Afraid? Excited? Try talking to your past self, and see if she replies. There is no script to follow, but this is a chance to see what comes from the older, deeper part of you.

You may find you don't get much from the exercise, or you might get some amazing insights into what you've been feeling. Get to know this buried part of you. Start a relationship with him or her, nurture it, and watch it grow.

Derek Draper was a Labour party lobbyist before suffering depression and retraining as a psychotherapist. He now has a private practice in Marylebone, London. His work on curing depression won a Mind award.

His new book, Life Support: A Survival Guide for the Modern Soul, is published on 2 April.